HARDENING FERTILIZATION AND NUTRIENT LOADING OF
CONIFER SEEDLINGS
R. KASTEN DUMROESE
Kas Dumroese is Research Plant Physiologist with the USDA Forest Service Southern Research Station,
1221 South Main Street, Moscow, ID 83843; telephone: 208.883.2324; email: kdumroese@fs.fed.us
Dumroese R.K. 2003. Hardening fertilization and nutrient loading of conifer seedlings. In: Riley L.E.,
Dumroese R.K., Landis T.D., technical coordinators. National Proceedings: Forest and Conservation
Nursery Associations—2002. Ogden, UT: USDA Forest Service, Rocky Mountain Research Station.
Proceedings RMRS-P-28: 31–36. Available at: http://www.fcnanet.org/proceedings/2002/dumroese.pdf
Abstract
Continuing to fertilize bareroot and container seedlings during the hardening process (from cessation of height growth
until lifting) can improve seedling viability. The process of fertilizing during hardening has many names, but in the
last decade a new term, nutrient loading, has come into use. The process of nutrient loading seedlings leads to luxury
consumption indicated by foliar nitrogen (N) concentrations > 2.5%. Increasing foliar nutrient concentrations during
hardening can result in increased cold hardiness, seedlings with thicker stems, heavier root systems, better root-toshoot ratios, higher foliar N concentrations and contents, and the potential for better survival and growth on the
outplanting site.
Key Words:
Loblolly pine, Pinus taeda, slash pine, Pinus elliottii, longleaf pine, Pinus palustris, irrigation, crop scheduling,
bareroot, container seedlings, water management, hardening
INTRODUCTION
Nursery managers are most concerned with levels of
nitrogen (N) in seedlings because this nutrient fuels
seedling growth rate. Traditionally, in bareroot and
container nurseries a standard operating procedure is
to reduce N applications once target height is
achieved (Tinus and McDonald 1979; Duryea 1984;
Landis and others 1989). The idea behind this is that
seedlings need to be hardened for outplanting, and
cessation of shoot growth is necessary for the
hardening process to begin. However, reducing N
applications while the seedling is still growing results
in a “dilution” of N within the plant—the N
concentration will decline because the N content is
spread throughout more tissue (see sidebar).
Seedling N content is usually expressed as a
weight, often in milligrams—it is the physical
amount of N within the seedling. The N
concentration, however, is N content divided by
biomass and expressed as a percentage. Both N
content and concentration can be determined for
particular seedling parts such as needles, buds,
stems, or combined together as shoots or roots or
even whole seedlings. For nursery stock, foliar N
concentration is the most commonly used value.
Realizing that seedling nutrient reserves are
important for survival and growth after outplanting,
nursery managers can begin to increase seedling
nutrient concentrations prior to harvesting. The
rationale is that these nutrients can be translocated
during the establishment phase until seedling root
systems can again provide the necessary nutrient
uptake. Such fertilization applications have often
been referred to as late-season, post budset, late
summer, early fall, or fall fertilization. In the past
decade, a new term has come into use: nutrient
loading.
Nutrient loading is the practice of applying large
doses of fertilizers to seedlings in order to allow
luxury consumption of those nutrients (fig. 1).
Luxury consumption is, simply, an increase in
nutrient concentration above what the seedling can
use for growth under optimum conditions. In conifer
seedlings, foliar N concentration ranges from about
0.8% to 3.5%, with the optimum range considered to
be from 1.5% to 2.5% (Landis and others 1989).
Therefore, using my definition, foliar N
concentrations > 2.5% are the result of nutrient
loading and indicate luxury consumption. Although
nutrient loading is usually done just prior to
harvesting, Dr VR Timmer advocates using this
technique throughout the growing season (see
31
Timmer and Aidelbaum 1996). His trials have been
mainly done with Canadian conifer seedlings with
determinant growth patterns (Timmer and Munson
1991; Miller and Timmer 1994).
Because of Dr Timmer’s prolific publishing on
nutrient loading, this phrase is being more commonly
used in bareroot and container nurseries. My
objective for the remainder of this paper is to review
a little of what we know about the interactions of
seedling growth, cold hardiness development, and
hardening and nutrient loading fertilization of
conifers in general, and southern pines in particular.
NITROGEN AND CONIFER SEEDLING COLD
HARDINESS
One justification for reducing N late in the growing
season is to stimulate cold hardiness development.
Unfortunately, reducing N too much may actually
suppress development of cold hardiness (fig. 2).
Timmis (1974) found that Douglas-fir (Pseudotsuga
menziesii) seedlings with low N concentration (0.8%)
had a LT50 (the temperature at which 50% of the
population died) of 9 °F (–13 °C) but those with a
higher concentration (1.6%) were much more cold
hardy (–22 °F [–30 °C]). Similarly, red spruce (Picea
rubens) with 2.3% foliar N concentration were as
cold hardy as those with only 0.6% N (Klein and
others 1989) and in a study with many types of
plants, Pellet and others (1981) found that plants with
luxury amounts of N hardened as well as those with
optimum N and better than those with N deficiency.
32
-2
Frost hardiness (LT50), °C
Figure 1. Growth increases with increasing nutrient levels
up to a critical point (A), after which, increasing nutrient
levels do not result in more growth, but lead to luxury
consumption. Nutrient loading is a new term for luxury
consumption. Source: Landis and others (1989).
Well-fertilized Scots pine (Pinus sylvestris) suffered
less frost damage than nutrient-deficient seedlings
(Rikala and Repo 1997), and Norway spruce (Picea
abies) seedlings showed similar frost damage despite
a range of foliar N concentrations between 2.2% and
3.3% (Floistad 2002). Sugar metabolism, and
therefore presumably other seedling physiological
processes, were not affected by late season N
applications to loblolly pine (Pinus taeda) in Georgia
(Sung and others 1997). Hansen (1992) found that
prolonged fertilization of Japanese larch (Larix
leptolepis) did not translate into low cold hardiness
development. Coastal Douglas-fir seedlings, given an
additional 285 lb N/ac (320 kg N/ha) over a 10-week
period from mid-September to early November, had
the same cold hardiness as nonfertilized seedlings
(Birchler and others 2001).
It is important to realize that these fertilization
practices were not designed to “push” seedlings to
continue height growth late into the season, but
rather to maintain or enhance foliar N concentrations.
Naturally, we are all aware that conifer seedlings
with active height growth are very susceptible to
frost injury. Therefore, the difficulty in nutrient
loading is balancing N inputs while avoiding bud
break or shoot extension.
-4
Aug 7
-6
Aug 21
Sep 4
-8
-10
Sep 18
-12
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Foliar N concentration (%)
Figure 2. At each sampling date, seedlings receiving the
highest rate of fertilizer (triangles) had the highest foliar N
concentration and were more cold hardy than seedlings
receiving the least amount of fertilizer (circles). Source:
Rikala and Repo (1997).
In reading through the following studies on fall
fertilization of southern pine species, note that none
of them truly addresses foliar concentrations > 2.5%
N. In other words, no nutrient loading occurred.
Despite this lack of nutrient loading, benefits to
fertilizing in fall were still realized.
Bareroot Slash Pine
Slash pine (Pinus elliottii) seedlings grown in Florida
with 2 applications of 150 lb/ac (168 kg/ha)
ammonium nitrate (51 lb N/ac [ 57 kg N/ha]) and
receiving an additional 150 lb/ac ammonium nitrate
in late August were taller and had larger root collar
diameter (RCD), root dry weight, and foliar N
concentration at lifting in January than seedlings that
did not receive supplemental fertilization (Duryea
1990). Seedlings fertilized much later in the season
(early November) with 75 lb/ac (84 kg/ha)
ammonium nitrate had similar morphology but foliar
N concentration and content were higher. After the
first year in the field, August-fertilized seedlings
were taller than the controls, but this characteristic
failed to persist after 3 growing seasons (Duryea
1990).
Subsequently in Florida, Irwin and others (1998)
grew seedlings in beds incorporated with 300 lb/ac
10N:10P2O5:10K2O (30 lb N/ac [34 kg N/ha)]) and
later topdressed with 5 ammonium sulfate
applications at various rates during the growing
season (mid June to mid August; 143 lb N/ha [160 kg
N/ha] total applied). Seedlings were fertilized 1 or 3
times between mid November and mid December
with 51 lb N/ac (57 kg N/ha) ammonium nitrate. A
month later (mid January) seedlings were lifted. The
1 to 3 additional fertilizations had no effect on RCD,
height, bud length, dry weight, root:shoot, premature
bud break, days to budbreak, or the number of lateral
roots. Foliar N concentration was higher, however, in
all fertilizer treatments and N content was higher in
the roots, stems, and needles of seedlings given 3 late
season fertilizer applications (fig. 3). Foliar N
concentration ranged from 1.8% in the 3X fertilizer
treatment to 1.4% in the 1X treatment and 1.1% in
the control. Not only are none of these treatments
nutrient loading, the 1X fertilization and the control
are below optimum levels. Seedlings with the higher
N concentrations, however, had higher survival,
more height growth, and thicker stems after the first
field season than control seedlings.
2.0
Foliar N concentration (% dry mass)
DOES NUTRIENT LOADING APPLY TO
SOUTHERN PINES?
High -- seedlings fertilized
Nov 15, Nov 29, Dec 13
1.8
1.6
Low -- seedlings fertilized
Nov 15
1.4
1.2
None -- seedlings not
fertilized
1.0
0
2
4
6
8
Time after first fertilization
(weeks)
Figure 3. Slash pine seedlings without fertilization (None)
continued to grow, but nitrogen (N) content remained the
same resulting in the dilution of foliar N from 1.3% to
1.1% over a 2-month interval. Although seedlings in the
Low treatment showed an initial increase in foliar N
concentration after being fertilized, after 1 month seedling
growth again outpaced N uptake and dilution occurred,
with concentration dropping from 1.6% to 1.4%. None of
these seedlings were nutrient-loaded. Source: Irwin and
others (1998).
Bareroot Longleaf Pine
During spring and early summer, longleaf pine
(Pinus palustris) seedlings were given small periodic
applications of ammonium nitrate totaling about 100
lb N/ac (112 kg N/ha). In late October, seedlings
were fertilized once with a variety of fertilizers, but
always with 150 lb N/ac (168 kg N/ha). Fertilized
seedlings had 20% more dry weight with roots and
buds accounting for three-fourths of that increase.
RCD was 11% greater and subsequently root:shoot
was increased. Foliar N content averaged 60% more
than nonfertilized controls. The average foliar N
concentration of fertilized seedlings was 1.4% versus
1.1% in the control. Again, neither of these
treatments were nutrient loading and both resulted in
less than optimal foliar N concentration. The result,
however, was a trend toward fall fertilization
decreasing the time the seedlings spent in the grass
stage and better survival after 8 years in the field
(Hinesley and Maki 1980).
Bareroot Loblolly Pine
One study with loblolly looked at the effects of foliar
N concentration on subsequent seedling performance
in the field (Larsen and others 1988). Data from 20
33
nurseries indicated the average foliar N concentration
was only 1.7%, a value at the low end of the
optimum range (1.7% to 2.3%) suggested by Fowells
and Krauss (1959). This low average value was
unfortunate because Larsen and others (1988) found
that foliar N content was positively correlated with
initial and subsequent field growth, and in fact, was
the only variable after 3 years in the field that was
significantly correlated with both diameter and
volume growth—seedlings with higher N content
performed better than those with low content. A
modest application of 41 lb N/ac (46 kg N/ha) in mid
September in a southern Georgia nursery raised foliar
N concentration from 1.2% to 1.6% (Sung and others
1997). The fertilized crop had fewer culls and more
first-order lateral roots, thicker stems, and higher dry
weights of roots and shoots. Further, fertilization had
no effect on sugar metabolism or the pattern of dry
matter allocation. Recently, South and Donald (2002)
report that applications of N and phosphorus one
month before lifting increased foliar N concentrations
up from 2.0% to 2.4% which translated into taller
seedlings, higher volume production, and better
survival 5 years after outplanting. Again, none of
these studies involved nutrient-loaded seedlings but
show that fall fertilization can increase seedling
viability.
Container Seedlings
Although the container industry is relatively young in
the South and mostly restricted to longleaf pine
(although interest is mounting in growing loblolly
and slash in containers too), it is important to
recognize that fertilization during hardening can
greatly improve seedling viability.
For container nursery managers, more methods are
available for applying nutrients: conventional, foliar,
and steady-state. Conventional fertilization, applying
soluble fertilizers through the irrigation system, is the
most straight-forward way of applying nutrients. In
Scots pine, late-season fertilization increased RCD
and resulted in more cold-hardiness development
(Rikala and Repo 1997).
In addition, foliar fertilizers, specially formulated for
application onto needles at high rates, can be used to
quickly recharge nutrient-depleted seedlings or to
add high doses of nutrients for luxury consumption.
An advantage of foliar fertilization is that nutrients
can be applied to the crop without adding water to
the medium, allowing the grower to keep seedling
growth under control through water management
while still adding nutrients directly to the crop. This
has been demonstrated in a crop of ponderosa pine
(Pinus ponderosa), where Montville and others
(1996) used foliar fertilizer during the bud initiation
phase to concurrently cease height growth, grow
buds, and avoid foliar N concentration dilution—the
benefit was a 45% increase in RCD (fig. 4).
The theory of steady-state nutrition is that during
production, seedling foliar N concentration is
maintained at the same level. Much of the literature
on this deals with red pine (Pinus resinosa), black
Foliar N concentration (%)
2.5
Caliper
42 %
2.0
High
a
Med
1.5
b
Low
c
d
Control
1.0
Bud initiation
Budset
Time
Season end
0.9
1.0
1.1
1.2
1.3
1
Growth Increment
Figure 4. At bud initiation, seedlings received 3 rates of foliar fertilizer with a nonfertilized control. Note
the dilution of N in the control seedlings during the bud initiation treatment. Avoiding N dilution
increased RCD (caliper) growth—higher rates of fertilizer resulted in higher RCD increments (cm)
compared to the control. Source: Montville and others (1996).
34
spruce (Picea mariana), and white spruce (Picea
glauca) in Canada. The benefits of maintaining
optimum or luxury foliar N concentrations (3.0%+),
particularly late in the growing season, include
increased root:shoot, N accumulation in roots, and
height and biomass growth on the outplanting site
(Timmer and Miller 1991; Timmer and others 1991;
Miller and Timmer 1994; Malik and Timmer 1995;
McAlister and Timmer 1998).
light quality, feedback from customers, anticipated
conditions on the planting site, and the experience
and philosophy of the nursery manager. Elucidating
“the” fertilizer regime that will work universally well
in all nurseries is impossible (Dumroese and Wenny
1987). With that in mind, however, I hope you are
encouraged to reexamine the use of hardening
fertilization toward the goal of improving seedling
viability.
CAUTION—ANIMAL DAMAGE
ACKNOWLEDGMENT
Animal damage is a problem on many plantations—
the literature is full of “animals ate our seedlings”
stories. Further, plenty of anecdotal information
exists from plantations that animals prefer “normal”
nursery seedlings over naturals, presumably because
of their higher nutritional value. Unfortunately, I
could not find any published information on the
correlation between foliar N concentration and
herbivory in seedlings. In young plantations, however,
fertilization resulted in increased animal damage
(Sullivan and Sullivan 1982) and in a plantation
where fertilization raised foliar N concentrations
20%, damage increased 6X (Gessel and Orians
1967). I think we should be concerned about nutrient
loading because it may exacerbate this problem on
some sites.
I thank Drs James Barnett, Thomas Landis, Deborah
Page-Dumroese, and David South for comments on
earlier drafts.
SUMMARY AND RECOMMENDATIONS
The only way to know seedling nutrient concentrations
is to measure them—sending in a few foliar samples
during the hardening process will enable you to know
if your crop has optimum nutrition. As seen from the
above discussion, seedling viability can be enhanced
by maintaining optimum nutrient concentrations
during the hardening process. For southern pines,
that probably does not involve true nutrient loading,
but adherence to a hardening fertilization program
that keeps foliar N concentrations in the optimum
zone (1.5% to 2.5%) and regulates height growth
through water management. At these optimum
nutrient levels, growers may expect normal
development of cold hardiness; improved RCD, bud
size, and root biomass (and therefore improved rootto-shoot ratio); and enhanced survival and growth
after outplanting.
It is important to remember that each nursery has its
own idiosyncrasies, and fertilizer regimes develop in
response to many variables, including species, seed
sources, irrigation system, water quality, nursery
elevation and subsequent microclimate, annual days
of sunshine, age of greenhouse roofs and subsequent
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